- generative test doesnt apply to mysql, others

- generative test doesnt apply to mysql, others
- refactoring to unitofwork.py.  low-hanging cruft
removed, UOWTask structure simplified particuularly with the
per-instance sort phase, most methods docstring'ed extensively.
this is a merge from the 'uowsimplify' branch.  (only slightly simpler, tho)
- mapper delete_obj works across multiple mappers to be consistent
with the operation of save_obj